System and method for monitoring and controlling the temperature of a catheter-mounted heater

ABSTRACT

A system for monitoring and controlling the temperature of a heating element in a thermodilution catheter to maintain it within safe physiological limits includes, in the preferred embodiment, a thermodilution catheter having a resistive heating element with a known temperature coefficient of resistance and system elements which (1) supply and control power to the heating element, (2) monitor the temperature of the heating element by monitoring its resistance, (3) determine the rate of change of the temperature of the heating element, and (4) respond to the rate of temperature change measurement by removing power from the heating element. In an alternate, preferred embodiment, the system further comprises a calibration circuit or resistor and circuit elements which (1) control the supply of power to the system, (2) switch power between the calibration circuit or the heating element, (3) determine whether power has been supplied to the calibration circuit, and (4) respond to the power determination to remove power from the system. The system also includes methods of operation for each of the above-referenced system embodiments.

RELATED CASES

This application is a continuation-in-part of U.S. application Ser. No.08/268,217, filed on Jun. 29, 1994, for Electrical Power Amplifier ForContinuous Cardiac Output Monitoring, which is incorporated herein byreference as though fully set forth.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to thermodilution catheters of the typethat have a heating element which applies heat to a patient's blood forthe purpose of measuring a physiological condition, such as volumetricblood flow. More specifically, the invention relates to systems andmethods for controlling the application of power to the heating elementin order to prevent harm to the patient or damage to the catheter.

2. Description of the Prior Art

It has recently become known in the art that cardiac output can becontinuously measured by utilizing a heating element on a thermodilutioncatheter. By applying a known thermal signal on a regular or continuousbasis, cardiac output measurements can be made continuously by detectingthe thermal signal downstream from the input signal and by applyingsophisticated signal processing techniques. One such cardiac outputmeasurement system is disclosed in U.S. Pat. No. 4,507,974 to Yeldermanwhich discloses the use of a heat signal which is generated according toa pseudo-random binary sequence (PRBS). Correlation techniques are thenused at the downstream position to extract the flow rate of the blood.An improved version of this system is disclosed in U.S. Pat. No.5,146,414 to McKown, et al. The McKown et al. patent discloses arecursive or adaptive processor and uses the lagged normal model for thecardiac system in order to enhance the signal to noise ratio of thesensed downstream thermal signal.

In order for a heating element to be utilized on or in a catheter thatis to be placed in a human body, certain safety requirements must bemet. Most importantly, the heating element must be controlled to insurethat it is operating within certain acceptable temperature ranges.Additionally, certain fail-safe mechanisms must be utilized to insurethat the heating element does not get too hot. In addition to the normaltemperature controls, it would be useful to be able to determine whenthe heating element is either subject to low flow conditions, operatingin air, or otherwise under operating conditions wherein any excess heatis not being adequately dissipated.

One such prior art thermodilution heating element catheter utilizes aresistive heater having a known thermal coefficient of resistance. Bymonitoring the patient's blood temperature and the heating elementresistance it is possible to monitor and automatically control thetemperature and heat output of the heating element temperature. Certainembodiments of such a resistive heater thermodilution catheter andheater control system are disclosed in the following related U.S. PatentApplications: Ser. No. 08/049,231, to Quinn, et al., Ser. No.08/245,727, to Yelderman et al., and Ser. No. 08/334/443, to McKown etal, (hereinafter collectively referred to as Quinn et al.) assigned toInterflo Medical, Inc., and incorporated herein by reference as thoughfully set forth. In alternate systems, a thermistor or thermometer isused in conjunction with the heater in order to continuously monitor thetemperature of the heating element.

The catheter heating element disclosed in Quinn et al, has proven to bevery safe in that it provides a means to continuously monitor andautomatically control the application of power to the heating element,thereby controlling its temperature. Certain fail-safe systems have thusbeen incorporated into a heat-based continuous cardiac output systemwhich utilizes the teachings of Quinn et al. An example of such a systemwas marketed in the VIGILANCE® cardiac output monitor marketed and soldby Baxter Healthcare Corporation, a subsidiary of Baxter International,Inc., the assignee of the present invention. The VIGILANCE® systemincorporates certain fail-safe mechanisms in order to avoid theapplication of excessive heat to the patient's bloodstream. Thesefail-safe mechanisms come into play when the heating element rises abovea certain predetermined temperature level.

Although the use of the Quinn et al. catheter in conjunction with theVIGILANCE® system has proven to be safe for human clinical use, therewould be further utility in being able to determine, more swiftly andredundantly, when the heating element catheter is operating in very lowflow conditions or in "no flow" conditions outside of the human body.Although the VIGILANCE® system provides for reducing power to theheating element when it reaches or exceeds its predetermined temperaturelevel in order to prevent damage to the patient, it would be useful toprovide a redundant control of the heating element under low flowconditions while the catheter is present in the patient's body.

It would also be useful to reduce power to the heating element while thecatheter is outside the patient's body. For example, just prior tocalibration of the VIGILANCE® system, power is supplied to a calibrationcircuit. If power is inadvertently supplied to the heating element whilethe catheter is under a zero flow condition outside the patient's body,it could result in thermal deformation in the catheter tubing in theregion of the heating element.

It would also be useful to know, while the catheter was in the patient'sbody, that power being applied to the system was not going to theheating element, but was instead going to other parts of the system,such as the calibration circuit or resistor.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to provide a system formonitoring and controlling the temperature of a thermodilution catheterheating element within safe physiological limits and under variousconditions and environments.

It is a further object of the invention to automatically reduce power tothe heating element under low flow or zero flow conditions.

It is a further object of the invention to reduce power to the catheterheating element while the catheter is in the patient's body if thetemperature of the heating element is rising in such a fashion as toindicate that it will reach a predetermined maximum level.

It is a further object of the invention to reduce power to the systemwhile the catheter is in the patient's body if power is beingunintentionally applied to the calibration circuitry instead of theheating element.

It is a further object of the invention to reduce power to the heatingelement if power is unintentionally provided to the heating elementduring a calibration regimen while the catheter is outside the patient'sbody.

To achieve the above-referenced and other objects of the invention notspecifically set forth, a system according to a first aspect of theinvention for controlling power to the heating element of athermodilution catheter comprises a thermodilution catheter having aheating element, means for supplying and controlling power to theheating element, means for monitoring the temperature of the heatingelement, means for determining the rate of change of the temperature ofthe heating element and means responsive to the rate of temperaturechange measurement to control the power supplying means to remove powerfrom the heating element.

In one preferred embodiment, the heating element is a resistive heaterwith a known characterized resistance and a known temperaturecoefficient of resistance and the temperature monitoring means comprisesmeans for monitoring the resistance of the heating element. In anotherpreferred embodiment, the monitoring means further comprises means formeasuring the current applied to the heating element. Alternatively, theheating element could be monitored by having a dedicated thermistor orthermometer which is used in conjunction with the heating element.

According to a second aspect of the invention, a system for controllingpower to the heating element of a thermodilution catheter comprises athermodilution catheter having a heating element, a calibration circuit,means for controlling the supply of power to the system, means forswitching power between the calibration circuit or the heating element,means for determining whether power has been supplied to the calibrationcircuit, and means responsive to the calibration circuit monitoringmeans to remove power from the system.

In a preferred embodiment, the calibration circuit is a resistor, thepower supply switching means comprises a switch between the calibrationcircuit and the heating element, and the means for monitoring the supplyof power to the calibration circuit is means for sensing the voltageacross the resistor. In an alternate, preferred embodiment, the meansfor monitoring the supply of power to the calibration circuit furthercomprises means for determining the rate of change of the temperature ofthe heating element.

A method according to a third aspect of the invention for controllingpower to the heating element of a thermodilution catheter includes thesteps of (1) supplying and controlling power to the heating element, (2)monitoring the temperature of the heating element, (3) determining therate of change of the temperature of the heating element, and (4)controlling the power supplied to the heating element in response to therate of temperature change determination.

A method according to a fourth aspect of the invention for controllingpower to the heating element of a thermodilution catheter includes thesteps of (1) supplying power either to the calibration circuit or theheating element, (2) monitoring the supply of power to the calibrationcircuit, and (3) removing power from the system in response to monitoredcalibration circuit.

These and various other advantages and features of novelty whichcharacterize the invention are pointed out with particularity in theclaims annexed hereto and forming a part hereof. However, for a betterunderstanding of the invention, its advantages, and the objects obtainedby its use, reference should be made to the drawings which form afurther part hereof, and to the accompanying descriptive matter, inwhich there is illustrated and described certain preferred embodimentsof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a fragmentary and somewhat schematic view of a humanpatient with a thermodilution catheter having a heating element placedin the patient's right ventricle, and a monitoring apparatus associatedwith this catheter;

FIG. 2 is a schematic depiction of the functional interaction betweenthe thermodilution catheter and the modular embodiment of the controlsystem of the present invention;

FIG. 3 is a schematic block diagram of a prior art thermodilutioncatheter heating element control system;

FIG. 4 is a schematic block diagram of an exemplary, preferredembodiment of the thermodilution catheter heating element control systemof the present invention;

FIG. 5 is a graph of current versus time curves illustrating thedifferences between a prior art control system and the control system ofthe present invention;

FIG. 6 is a schematic block diagram of another exemplary, preferredembodiment of the thermodilution catheter heating element control systemof the present invention;

FIG. 7 is a schematic block diagram of yet another exemplary, preferredembodiment of the thermodilution catheter heating element control systemof the present invention;

FIG. 8 is a schematic block diagram of yet another exemplary, preferredembodiment of the thermodilution catheter heating element control systemof the present invention;

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

A system in accordance with preferred exemplary embodiments of theinvention will be described below in detail with reference to FIGS. 1-8.It will be appreciated by those of ordinary skill in the art that thedescription given herein with respect to those embodiments is forexemplary purposes only and is not intended in any way to limit thescope of the invention. All questions regarding the scope of theinvention may be resolved by referring to the appended claims.

A detailed description of pulmonary artery catheters, and thermodilutioncatheters in particular, is not given herein, for the features of suchcatheters are well known to those familiar to the art. FIG. 1 shows ahuman patient 10, within whose right ventricle 12 has been placed athermodilution catheter 14 having a heating element 16. The catheter hasa flotation balloon 18 at its distal end and it is placed according totechniques known to those skilled in the art. Externally of the patient10, the catheter 14 is connected to a monitor 20 which includes adisplay screen 22 upon which information about the patient's conditioncan be displayed. In a preferred exemplary embodiment, as shown in FIG.1, catheter 14 is connected to a module 24, which contains thecontinuous cardiac output ("CCO") measurement subsystem. Module 24 canbe inserted into various monitors such as monitor 20, which isconfigured to receive various modules 24, each of which is adapted tomonitor certain patient physiological parameters. The modularity of theCCO measurement subsystem allows it to be moved around the hospital tovarious bedside locations. The configuration of monitor 20 allowing itto receive various modules permits various types of patient informationto be integrated into one monitor and also permits such information tobe provided to a remote location, such as to a nurse's station, centralpatient monitoring and data recording computer system, or to a physicianwho may wish to receive the information at his home or office via atelephone line interconnection with such a hospital's central patientmonitoring computer system.

With attention now to FIG. 2, which illustrates the modular embodimentof the present invention, it is seen that module 24 has connection tomonitor 20 via a data bus, generally indicated by the double-headedarrow 26, and via a number of electrical connections which supplyelectrical power to module 24 from monitor 20. It will be appreciatedthat FIG. 2 is very schematic, and that the electrical conductorsdepicted outwardly of catheter 14 are actually of a fine gauge and overa portion of their length are disposed within the elongate andcomparatively thin shaft of the catheter. Another portion of the lengthof the illustrated conductors will be understood to be provided by cable28. In this embodiment, two of the conductors 30 and 32 connect toheating element 16 which is outwardly disposed on the distal end portionof catheter 14. In a preferred, exemplary embodiment, heating element 16may be configured as a flexible thin metallic film element having aknown impedance characteristic and a high coefficient of resistancechange with change in temperature. Catheter 14 will preferably beconfigured so that heating element 16 is actually disposed in the rightventricle of patient 10. The turbulent blood flow in this ventricleresulting from the pumping action of the heart assists in distributingheat energy from heating element 16 uniformly throughout the pulmonaryblood flow. Downstream of heating element 16 with respect to thedirection of blood flow (indicated with arrows 34) is disposed atemperature measuring sensor 36. Sensor 36 may be a small beadthermistor, for example, and is connected to cable 28 and monitor 20 viaconductors 38 and 40.

Within module 24, conductors 38 and 40 supply the temperature signalfrom sensor 36 to a microprocessor based control system 42, whichincludes microprocessor 44 and sense circuit 45. Sense circuit 45converts output voltage and current into digital data for microprocessor44. In a preferred, exemplary embodiment, sense circuit 45 includes adifferential amplifier and an RMS convertor (not shown).

In a preferred, exemplary embodiment, control system 42 includes poweramplifier circuit 46. Microprocessor 44 has a two-way control and datainterface with power amplifier circuit 46, as is generally indicated bythe control and data bus arrow 48. This general interface referencenumeral (48) will be used in various places throughout the followingexplanation to refer to the interface of information and control signalsin one or both directions between power amplifier 46 and microprocessor44.

In the prior art system of Quinn et al, referenced above, the heatingelement communicates with a cardiac output computer which receives powerlevel signals for controlling the heating element. By using a heatingelement comprised of material which has a high temperature coefficientof resistance, not only can it be used as a heat supplier, but it canalso serve as its own temperature sensing device according to theprinciples outlined in Quinn et al.

By using a power source of a controlled voltage amplitude, and bymeasuring blood temperature, an increasing catheter heating elementtemperature can be directly detected as an increasing heating elementresistance which reduces the power delivered to the heating filament. Inthis manner, the actual current and voltage to the catheter heatingelement can be continuously monitored and thereby controlled. Thus,during operation the cardiac output computer may continuously measureand monitor the temperature of the heating element to keep it within apredetermined safe limit.

FIG. 3 illustrates a prior art control system which embodies theteachings of Quinn et al. This system was embodied in the VIGILANCE®cardiac output monitor marketed and sold by Baxter HealthcareCorporation, a subsidiary of Baxter International, Inc., the assignee ofthe present invention. As seen in FIG. 3, drive amp 50 drives a desiredsinusoidal signal across isolation transformer 52 to the isolatedpatient-connected section of the system which includes heating element16 of catheter 14, and a calibration resistor 54. The isolatedpatient-connected section includes relay control 55 which can send thesignal either to heating element 16 or to calibration resistor 54 viarelay 56.

In order to effectively control the heating element temperature, theVIGILANCE® cardiac output system includes a current monitoring subunitand a voltage monitoring subunit supplied to heating element 16. Thevoltage monitoring subunit is connected across the primary winding 58 ofisolation transformer 52 and comprises differential amp 60, RMSconvertor 62 and microprocessor controlled, variable gain, filteredpotentiometer 64.

Differential amp 60 in the voltage measuring subunit is used to scalethe signal down to an appropriate voltage level for monitoring purposes.RMS convertor 62 then demodulates by converting the AC signal to a DCsignal. The signal is again scaled by variable potentiometer 64 whichsends it to multiplexer switch 66. Switch 66 sends the signal to Analogto Digital Convertor ("ADC") 68 where it is converted to a digitalsignal and sent on to microprocessor 44 (not shown).

The current monitoring subunit is connected across sense resistor 70 todraw current for monitoring purposes and comprises differential amp 72,RMS convertor 74 and low pass gain stage 76. In the VIGILANCE® cardiacoutput monitor, gain stage 76 includes a low pass filtered inverting opamp circuit with a programmable resistor. Differential amp 72 is used toscale the signal up to an appropriate voltage level for monitoringpurposes. RMS convertor 74 then demodulates by converting the AC signalto a DC signal. Gain stage 76 scales the signal and sends it tomultiplexer switch 66. As described above with respect to the voltagemonitoring subunit, the signal is sent to ADC convertor 68 where it isconverted to a digital signal and sent on to microprocessor 44 (notshown).

As discussed above with respect to the Quinn et al. system, theaforementioned control system provides a fail-safe control wherein thetemperature of heating element 16 can be monitored such that power canbe removed from heating element 16 when some absolute temperaturethreshold is met. However, there are circumstances where it would behelpful to also monitor the rate of change of the heating elementtemperature in order to provide a either a fault tolerant or a fasterfail-safe system.

Furthermore, during a calibration regimen, relay 56 sends the signal tocalibration resistor 54 to calibrate the system. It would be helpful tobe able to determine whether relay control 55 has failed and whetherpower has been supplied to heating element 16 during the calibrationregimen. This is not always a safety issue per se, since manycalibration regimens occur prior to catheter 14 being inserted intopatient 10. However, if power is applied to heating element 16 whilecatheter 14 is in air, the catheter tubing could be thermally deformed.The prior art system of Quinn et al. includes means to determine whetherthe catheter is in air by measuring the temperature of heating element16 to determine whether it exceeds some absolute value for more than aspecified integrated time temperature product. This is done, however,without regard to whether power is being applied to calibration resistor54.

FIG. 4 illustrates a exemplary, preferred embodiment of thethermodilution catheter heating element control system of the presentinvention. This embodiment comprises means for determining the rate ofchange of the temperature of the heating element. In particular, thisembodiment utilizes dual bandwidth sensing circuit 78 which comprisesgain stage 76 having a particular rise time or bandwidth and unfilteredfixed gain circuit element 79 having a faster rise time or higherbandwidth than gain stage 76. Fixed gain element 79 scales the currentsignal coming from RMS convertor 74. Fixed gain element 79 couldcomprise a non-filtering inverting op amp. This would provides a currentsignal with an appropriate bandwidth to permit microprocessor 44 tocalculate the rate of change of the temperature of heating element 16.Alternatively, those skilled in the art would recognize that othercircuit elements could be substituted for the op amp, such asmultipliers, transistorized amplifiers, and filters, to generate theappropriate signal(s). By calculating the rate of change of temperatureof heating element 16, control of the heating element can beaccomplished in a shorter period of time. Specifically, if the rate ofchange exceeds a certain predetermined value, the system can withdrawpower from heating element 16 prior to it reaching its predeterminedtemperature threshold. The present invention accomplishes this withoutthe need for a blood temperature measurement and without a systemcalibration. This was not possible in the prior art Quinn et al. systemdue to the structure of the current monitoring subunit.

The advantages of the aforementioned embodiment of the present inventionwill become more evident upon review of FIG. 5, which graphicallyillustrates sensed current (i) versus time (t) curves for the prior artsystem and for the present invention. Curves 80 and 82 graphicallyrepresent the sensed current/time relationship in the prior art system.Since the system utilizes a constant voltage source and a resistiveheater, any change in current would indicate a change in resistance,since, given a constant voltage, the current and the resistance areinversely proportional. Furthermore, a change in resistance valueindicates a change in temperature, since resistance and temperature aredirectly proportional, as a consequence of the positive coefficient ofresistance of the heating element. Hence, any reduction in current fromthe threshold value would indicate an increase in temperature. In curves80 and 82 the sensed current rises to a threshold value and thenstabilizes. Since curves 80 and 82 indicate a constant current uponreaching the threshold value, the heating element temperature appears toremain constant. However, curve 82 indicates a lower current value whichis indicative of a higher temperature for heating element 16. In thiscase the heating element temperature has reached the maximum operatingtemperature without providing any advance warning.

Curves 84 and 86 graphically illustrate the sensed current/timerelationship in the present invention. Curve 84 illustrates thesituation wherein the sensed current rises to a threshold value and thenstabilizes. Again, since curve 80 indicates a constant current uponreaching the threshold value, the heating element temperature appears toremains constant. Curve 86 graphically illustrates the sensedcurrent/time relationship in the present invention wherein there is anoticeable drop in the current, indicating a precipitous rise in thetemperature of heating element 16.

A review of curves 82 and 86 will illustrate important differencesbetween the prior art system and the present invention. Both curvesreach a particular current value, at about 250 msec, which indicates amaximum temperature for heating element 16. However, curve 82 reachedits current level gradually and gave no advance warning. Curve 86, onthe other hand, began its descent around the 100 msec point. Curve 86provided rate of temperature change information which would permitmicroprocessor 44 to control the provision of power to heating element16. Thus, power could be withdrawn from heating element prior to thetime it reached it maximum allowable value.

The teachings herein will enable those skilled in the art to make otherembodiments which fall within the principles of the present invention.For example, a patient side circuit which functions to obtain a highbandwidth current signal could also be utilized to derive a rate ofchange temperature measurement along the lines discussed above.

Another exemplary, preferred embodiment is illustrated in FIG. 6. Thisembodiment is particularly suited to the module version of the CCOmeasurement subsystem illustrated in FIG. 2. FIG. 6 illustrates poweramp 46 of FIG. 2 in greater detail. Power amp 46 comprises variablevoltage source 88, frequency source 90 and switch-mode amp section 92.In other embodiments, a linear amplifier could be used in place of ampsection 92. As indicated above, arrows 48 indicate interfacing withmicroprocessor 44. The functioning of these components is more fully setforth in parent U.S. application Ser. No. 08/268,217, which has beenincorporated herein by reference as though fully set forth. Switch-modeamp section comprises current sense 94 which interfaces withmicroprocessor 44 along interface 48. Current sense 94 functions in asimilar fashion as fixed gain circuit element 79 of FIG. 4 to provide acurrent signal with an appropriate bandwidth to permit microprocessor 44to calculate the rate of change of the temperature of heating element16. In an exemplary, preferred embodiment, current sense 94 includes adifferential amplifier and an RMS convertor with sufficient precisionand bandwidth to generate the appropriate signal without variable gainelements.

In an alternate embodiment (not shown) the temperature of heatingelement 16 is sensed separately with a contiguous thermometer orthermistor dedicated for the purpose. In this embodiment, thetemperature of the heating element would be continuously monitored bythe thermometer or thermistor and this information would be provided tomicroprocessor 44 for calculation of the rate of change of the heatingelement temperature for an early warning of a rising heating elementtemperature and further control of the provision of power along thelines discussed above. Furthermore, the use of a resistive heatingelement could be used in conjunction with a contiguous thermometer orthermistor to provide a redundant monitoring capacity.

Another exemplary, preferred embodiment of the present invention isillustrated in FIG. 7. In this embodiment, which is yet anothermodification of the VIGILANCE® cardiac output monitor discussed above,voltage detector 96 senses voltage across calibration resistor 54 inorder to determine whether power has been transmitted to resistor 54 orto heating element 16. Those skilled in the art will appreciate thevarious circuit elements available to sense voltage across resistor 54.In an exemplary, preferred embodiment, a MOSFET or a Darlingtontransistor is used with a half-wave rectifier and filter.

The embodiment of FIG. 7 is particularly useful just prior to thecalibration of the VIGILANCE® system, wherein power is supplied to acalibration circuit while the catheter is outside of the patient's body.If there is a failure in relay control 56, power could inadvertently besupplied to heating element 16 while the catheter is under a zero flowcondition outside the patient's body, resulting in thermal deformationof the catheter tubing. By sensing the voltage across resistor 54, powerto heating element 16 can be removed, preventing damage to catheter 14.This embodiment also provides the information, while the catheter is inthe patient's body, that power being applied to the system is not goingto heating element 16, but is instead going to calibration resistor 54.

Yet another exemplary, preferred embodiment of the thermodilutioncatheter heating element control system of the present invention isillustrated in FIG. 8. This embodiment also relates to the moduleversion of the CCO measurement subsystem illustrated in FIG. 2. In thisembodiment, the voltage occurring across calibration resistor 54 is usedto drive transistorized voltage detector circuit 97 into conductivity.In this embodiment, circuit 97 utilizes Darlington transistor 98 with ahalf-wave rectifier and filter 99. Note that in this embodiment,calibration resistor 54 is in fact formed by two resistors connected inparallel. Conductivity at switching circuit 98 illuminates an LED 100.Light from LED 100 is beamed across patient isolation barrier 102 andilluminates photodiode 104, causing it to become conductive.Conductivity of photodiode 104 pulls low the signal on conductor 106,which has connection with microprocessor 44, as is indicated byinterface arrow 48. Thus, microprocessor 44 is informed that power isbeing dissipated through calibration resistor 54, and not heatingelement 16. This provides the same advantages discussed above withrespect to the embodiment of FIG. 7.

It is to be understood, however, that even though numerouscharacteristics and advantages of the present invention have been setforth in the foregoing description, together with details of thestructure and function of the invention, the disclosure is illustrativeonly, and changes may be made in detail, especially in matters of shape,size and arrangement of parts within the principles of the invention tothe full extent indicated by the broad general meaning of the terms inwhich the appended claims are expressed.

The invention claimed is:
 1. A system for monitoring and controlling thetemperature of a catheter mounted heating element, comprising:athermodilution catheter having a heating element; means for supplyingand controlling power to the heating element; means for monitoring thetemperature of the heating element; means for measuring the rate ofchange of the temperature of the heating element; and means responsiveto the rate of temperature change measurement to control the powersupplying means to remove power from the heating element.
 2. The systemof claim 1 wherein the heating element is a resistive heater with aknown characterized resistance and a known temperature coefficient ofresistance and said temperature monitoring means comprises means formonitoring the resistance of the heating element.
 3. The system of claim1 wherein said temperature monitoring means comprises a thermometer orthermistor contiguous to the heating element and connected to said meansfor measuring the rate of change of the temperature of the heatingelement to provide temperature information thereto.
 4. The system ofclaim 1 wherein said means for measuring the rate of change of thetemperature of the heating element comprises a current sensing means. 5.The system of claim 4 wherein said current sensing means comprises adual bandwidth sensing circuit means.
 6. The system of claim 5 whereinsaid dual bandwidth sensing circuit means further comprises a fixed gainstage circuit means.
 7. The system of claim 6 wherein said fixed gainstage circuit means comprises a non-filtering inverting op amp.
 8. Thesystem of claim 4 wherein said current sensing means comprises a sensingcircuit means having a bandwidth sufficient to measure the rate ofchange of temperature of the heating element.
 9. A method of monitoringand controlling the temperature of a thermodilution catheter mountedheating element, comprising the steps of:supplying and controlling powerto the heating element; monitoring the temperature of the heatingelement; measuring the rate of change of the temperature of the heatingelement; and controlling the supply of power to the heating element inresponse to the rate of temperature change measurement.
 10. The methodof claim 9 wherein the heating element is a resistive heater with aknown characterized resistance and a known temperature coefficient ofresistance and said temperature monitoring step further comprises thestep of monitoring the resistance of the heating element.
 11. The methodsystem of claim 9 wherein said temperature monitoring step comprisesproviding a thermometer or thermistor contiguous to the heating element.12. The method of claim 9 wherein said step of measuring the rate ofchange of the temperature of the heating element comprises the step ofsensing the current applied to the heating element.
 13. A method ofclaim 12 wherein said current sensing step comprises using a dualbandwidth sensing circuit to measure current applied to the heatingelement.
 14. A method of claim 13 wherein said current sensing stepfurther comprises using a fixed gain stage circuit means to measurecurrent applied to the heating element.
 15. The method of claim 12wherein said current sensing step comprises using a non-filteringinverting op amp to measure current applied to the heating element. 16.The method of claim 12 wherein said current sensing step comprises usinga sensing circuit having a bandwidth sufficient to measure the rate ofchange of temperature of the heating element to measure current appliedto the heating element.